1. Field of the Invention
This invention relates to endless belts for conveyors and, more particularly, to thermoplastic, toothed endless belts driven by pulleys.
2. Description of the Related Art
Low tension, direct drive conveyor belts are often used in situations where hygiene and cleanliness are critically important. For example, in food processing plants such as those that process meat products for human consumption, low tension, direct drive belt conveyors are used to transport items. Sanitation is critically important and, therefore, the endless belts used in such conveyors are conventionally made of materials that can be hygienically cleaned.
It is known to use thermoplastic belts with a smooth continuous surface on one side and teeth on the other side adapted to engage grooves or sheaves in a drive pulley, as shown for example in U.S. Pat. No. 5,911,307. But such a thermoplastic belt has characteristics of both a flat, stretchable belt that might be typically driven by a friction pulley, and a toothed belt driven by a drive pulley. These characteristics reflect the two basic ways that a drive pulley can transmit torque to the belt. In a flat belt, torque is transmitted to the belt through friction between the drive pulley surface and the adjacent surface of the belt. The effectiveness of this type of drive is a function of belt tension (both initial pretension and the tension generated due to the product load) and the coefficient of friction of the material of the belt surface and the material of the pulley surface. A friction driven flat belt is subject to contaminants that can affect the coefficient of friction. Moreover, elongated belts typically stretch over time and under load and such stretching can affect its tension. A thermoplastic belt in particular can stretch 3% of its length or more.
For these reasons, direct drive belts are preferred in such facilities as food handling operations. In an ideal toothed belt, torque is transmitted to the belt through the contact of a face of a tooth or recess on the pulley to a face of a tooth or recess on the belt. But the use of a thermoplastic toothed belt as a direct drive belt with a pulley introduces problems, primarily because of the elasticity of the belt.
Because a thermoplastic belt stretches under load, the belt teeth may not always mate with the pulley recesses or sheaves as the belt wraps around the pulley. Prior solutions have determined that the tooth pitch of the belt must be less than the pitch of the drive pulley at less than maximum elongation of the belt. Also, the pulley pitch must equal the pitch of the belt at maximum elongation, give or take a fraction of a percent. Moreover, to ensure that the belt teeth are positioned to enter the pulley sheaves, the width of each sheave in the pulley must exceed the belt tooth width at least by the amount of distance generated by elongating the belt the maximum allowable amount over the span of the belt wrap.
Yet problems remain in ensuring that the belt teeth stay engaged with the pulley sheaves over the full range of belt elongation and load in the field. Due to the necessary pitch difference between the belt and the pulley, only one belt tooth will be driven by a pulley sheave at any given moment. It has been found that this engaged tooth is always the tooth that is about to exit the pulley. For all subsequent belt teeth that engage the pulley sheaves at any given moment, there is a gap between the face of the belt tooth and the face of the pulley sheave, and that gap progressively increases in size for each successive tooth. The size of these gaps are a function of belt tension, in that each respective gap is largest when the belt has minimum tension and smallest when the belt is at maximum tension. If the belt tension exceeds a predetermined maximum, the entry tooth will no longer sit properly in the pulley sheave and effective drive characteristics will be lost. In other words, the pulley may rotate while the belt slips until a tooth engages again.
It can be seen that as the exiting tooth disengages from the drive pulley there remains some amount of gap between the following belt tooth and the face of its respective pulley sheave. Therefore, discounting any momentum of the belt and any friction between the belt and the pulley, the belt will effectively stop for a brief moment until the following sheave re-engages the new “exit tooth”. For this brief moment no torque is transmitted from the pulley to the belt and thus the belt speed is temporally retarded.
This motion causes a slight amount of vibration and noise in the system. Vibration increases in frequency as pulley tooth pitch is reduced and/or pulley rotation speed is increased. It may be nearly undetectable in belt applications with a small tooth pitch and a large amount of mass for damping, such as when large product loads approach a predetermined maximum for belt elongation. But for many applications, particularly where loads are light and/or belt speed is slower, the resultant vibration and noise may be unacceptable.
Nevertheless some slip between the belt and the pulley is what enables a direct drive application to work. This temporary disengagement of belt teeth from pulley sheaves causes the average belt speed to be less than the average pulley speed. In fact, the average belt speed is less than the pulley speed by the percentage of elongation that is still available in the belt (max elongation—current elongation). Because of this necessary slip, any characteristics of a flat belt drive will compromise the benefits of direct drive, e.g. friction. Friction between the belt and the pulley will retard slippage and may cause the trailing tooth to miss the pulley sheave altogether.
Another problem occurs when the belt is under virtually no tension. In some application such as a horizontally positioned conveyor, the weight of the lower span of the belt tends to pull the teeth at the exit point out of the respective pulley sheave. The critical area of belt wrap around the pulley is the short distance between the exit point and one pulley sheave pitch back. If the belt tooth remains engaged through this arc then proper drive will be achieved, but if not, belt teeth will “pop” and the driving dynamics will become uncontrolled.